1. Field of the Invention
This invention relates to an image forming apparatus, particularly an image forming apparatus to which multivalued image data is input for forming the data into a visible image which includes half-tones, and to a modulating method used therein.
2. Description of the Prior Art
A pulse-width modulating (PWM) method is known as a technique through which half-tones are expressed based upon an input multivalued image signal.
According to the conventional PWM method, a multivalued input signal is converted into an analog signal which is then compared with an analog waveform (usually a sawtooth waveform) used for comparison purposes, thereby converting the input signal into a pulse-width modulated signal.
This conventional PWM method will now be described with reference to the timing chart shown in FIG. 12.
In the conventional method, as shown in FIG. 12, an input multivalued input image signal VDO is converted into an analog voltage waveform by a D/A converter using an image clock signal VCLK transmitted in synchronism with the input image signal VDO, thereby producing an analog image signal AV.
A sawtooth waveform SAW for comparison purposes generated by an appropriate method and the analog image signal AV are compared by a comparator. If the voltage of the analog image signal AV is greater than the sawtooth waveform SAW for comparison, then an output signal OPD of the above-mentioned comparator is turned "ON". On the other hand, if the voltage of the analog image signal AV is less than the sawtooth waveform SAW for comparison, then the output signal OPD of the above-mentioned comparator is turned "OFF". As a result, the inputted multivalued input image signal VDO is converted into a PWM signal to produce the corresponding recording image signal.
However, with the conventional PWM method of this kind, unstable operation frequently occurs owing to noise and fluctuation of a reference potential for dealing with the analog signal. This makes it difficult to perform a stable conversion to a PWM signal.
Accordingly, an example of an expedient to solve this problem is a method in which the input image signal VDO is compared in the form of a digital signal without being converting into an analog signal. This method is implemented by a circuit having a construction of the kind shown in FIG. 13.
In FIG. 13, a master clock signal CLK is assumed to have a frequency that is n times that of the input image signal VDO.
The input image signal VDO is obtained in a latch circuit 21 in synchronism with an image clock signal VCLK obtained by frequency-dividing a master clock signal CLK using a frequency divider 23. The image signal VDO enters a comparator 24.
Numeral 26 denotes a comparison signal generator which generates a comparison signal CMPD for comparison with the input image signal VDO whenever the master clock signal CLK is produced. The number of bits in the signal CMPD corresponds to the number of bits in the input image signal VDO. The comparison signal CMPD output by the comparison signal generator 26 is synchronized with the master clock signal CLK in a latch 25 before being inputted to the comparator 24.
The comparator 24 compares the level of the input image signal VDO from latch 21 and the level of the comparison signal CMPD from the latch 25, and the result of the comparison is output as the output image signal OPD. When VDO&gt;CMPD holds, the comparator 24 turns the output image signal "ON". This signal serves as the recording signal.
The comparison signal CMPD enters the comparator 24 n times while one input image signal VDO is being applied to this comparator. Consequently, the output image signal OPD is a PWM signal having n times the amount of information possessed by the signal VDO.
FIG. 14 shows a time chart associated with the above-described circuit arrangement for a case where the input image signal VDO is a six-bit signal, n is four and an up/down counter is used as the comparison signal generator 26.
The up/down counter produces a level value obtained by adding or subtracting a predetermined value to or from the immediately preceding level value whenever the master clock signal is generated. The counter has upper- and lower-limit values decided for it. Subtraction starts when the upper-limit value is reached as a result of addition, and addition starts when the lower-limit value is reached as a result of subtraction. This operation is performed repeatedly to produce a pseudo-sawtooth waveform. With this method, items of data are compared with each other so there is no danger of unstable operation of the analog kind.
In the example of the prior art described above, however, the minimum pulse width of the output image signal OPD is decided by the frequency of the master clock CLK, and this is accompanied by a limitation upon the number of tones.
In order to increase the number of tones, the frequency of the master clock should be raised. However, electronic circuit elements have a limit upon their operating frequency, and therefore the upper limit of the usable frequency is determined by the electronic circuit elements employed. As a consequence, a PWM conversion having a high number of tones is difficult to carry out.
In general, high-frequency oscillators and high-speed electronic circuit elements having a high operating frequency limit are expensive, and it is uneconomical to construct all of the circuitry using high-frequency elements merely for the purpose of a high tonality PWM conversion.
A pulse-width modulating circuit is used in such image forming apparatus as laser-beam printers and LED printers.
FIG. 19 is a circuit diagram of a pulse-width modulating circuit in a conventional image forming apparatus, and FIG. 20 is an operation timing chart associated with the circuit of FIG. 19.
Four-bit multivalued image data received from external equipment (not shown) such as a host computer or scanner is loaded in a counter 61 at the leading edge of an image clock signal. The counter 61 is successively counted down by a count clock signal outputted by a count clock generator 62. When the counter output becomes zero, the counter 61 outputs a carry signal. In response, the Q output of a J-K flip-flop 63 is set at the leading edge of the image clock signal. The Q output is reset by generation of the carry signal. This output of the flip-flop 63 is a pulse-width modulated signal. This pulse-width modulated signal enters a laser driver circuit (not illustrated) to turn a laser element on and off, thereby sensitizing a photosensitive drum (not shown) so that half-tone printing may be performed using an electrophotographic technique.
However, in order to express a n-tone density using the conventional method described above, the count clock signal is required to have a frequency that is n times the frequency of the image clock signal. For example, if the image clock signal has a frequency of 1 MHz, a count clock signal having a frequency of 256 MHz is required in order to express 256 tones by an eight-bit multivalued image signal. This means that it is necessary to use a high-speed device such as costly ECL (emitter-coupled logic). Another problem is that radiation noise tends to be produced owing to the high-speed operation.
In a case where a half-tone image is output by a laser-beam printer employing electrophotography, a method is employed in which screen or dither processing or image processing such as pulse-width modulation is performed by a host computer, which has a data generating source, or a controller, etc., and binary-coded data is input to a printer engine section (printer).
In order to deal with the binary-coded data in accordance with this method, a high-efficiency, half-tone data transfer is performed by transfer data compression or the like. On the other hand, with regard to the depth direction of density, it is difficult to obtain the desired stabilized tones, despite the fact that the host computer or controller transmits the same half-tone image data, owing to a delay in the data transmission line, the conditions of the electrophotographic process and differences among equipment.
Furthermore, when different printers are employed using the same host computer or controller, etc., the fact that the correspondence between dither patterns and density differs depending upon the printer means that the host computer or controller requires density correction tables the number of which is equivalent to the number of printers connected. These tables conform to the printers used. A problem arises in it is difficult to achieve compatibility with the printers.
A method is available in which image data having tones, such as a document, is read in by an image reader and developed into a dot image to provide each dot with a value indicating thickness. FIGS. 23(a) and 23(b) are diagrams illustrating the input/output characteristics of a CCD. In a case where a CCD sensor or the like is used as the image input section of the image reader, the density information possessed by the original image is converted into substantially linear voltage information proportional to the light reflected from the original image, as shown in FIG. 23(a). Since the voltage information possesses a logarithmic relation with respect to density [FIG. 23(b)], this signal is subjected to a correction (a .gamma. correction) in the reader section. However, the image undergoes a major change depending upon the extent of the correction.
In addition, depending upon the model type, the host computer possesses various fonts employed by the particular manufacturer. Some host computers tend to express characters boldly, while others express characters more finely.
Thus, there are a large number of varying factors involved in reproducing image information. As a result, when a single system is constructed from such components as a reader, a host computer and a printer, various problems arise. For example, the obtained image may be too faint overall and characters may appear too fine. Conversely, the overall image may be too dark and the characters distorted. In extreme cases, characters appear fine and photographs or graphics appear distorted and without tone. Conversely, characters become too bold to be legible while photographs and graphics appear too faint.
A method that has been proposed to solve these problems involves controlling the exposure time per dot of the exposing beam by pulse-width modulation (PWM), thereby performing the equivalent of density modulation. Attempts have been made to overcome the foregoing problems by subdividing the density modulation steps in order to perform more faithful modulation of density.
However, in order to subdivide the density modulation steps, there is a great increase in the amount of processing performed by the pulse-width modulating section (especially the D/A converter contained in the PWM section). Furthermore, in the method of controlling exposure time per dot of the exposing beam by PWM, a variance in pulse width occurs in a region where pulse width is very small. As a consequence, the printing operation for low density becomes unstable.